From the Rebbe: The primary Beis Hamikdosh is in our own soul/heart, as it states, “Make for me a Beis Hamikdosh and I will dwell in them – in the soul/heart of everyone”, Hence when we arrange our own heart to be God’s holy Temple – allowing Him infinite space (which comes by humility, for ego takes up space – humility makes space) then God rebuilds the actual Holy Temple!